1. Field of the Invention
The present invention relates to a zoom optical system composed of plural lens groups and adapted for performing zooming by varying the distances between the respective lens groups in an optical axis direction, as well as to an imaging lens unit with the zoom optical system, and a digital apparatus loaded with the imaging lens unit.
2. Description of the Related Art
In recent years, with an explosive spread of a mobile phone and a personal digital assistant (PDA), compact digital still camera units or digital video units have been built in most of these apparatuses. In the apparatuses incorporated with the digital still camera units or the digital video units, a compact image sensor with a small number of pixels, as compared with an image sensor equipped in a digital still camera dedicatedly used for photographing, and an imaging lens unit equipped with a mono-focal optical system constituted of one to three plastic lens elements are generally used, considering severe constraints in size and cost required in such apparatuses.
Since the magnification of the mono-focal optical system is substantially in the same level as a human eye, an object to be photographed is required to be set as close as possible to the photographer. In current days when use of an image sensor with a large number of pixels and high performance has been progressed drastically, there is a demand for a compact zoom optical system that is compatible with an image sensor with a large number of pixels, enables a user (photographer) to photograph an object sufficiently away therefrom, and is loadable in a mobile phone or a like apparatus.
As an example of a compact zoom optical system, Japanese Unexamined Patent Publication No. 2001-21806 (hereinafter, called as “D1”) and U.S. Pat. No. 6,515,805 (hereinafter, called as “D2”) propose a zoom optical system of a so-called negative-positive two-unit type comprising a first lens group having a negative optical power, and a second lens group having a positive optical power in this order from the object side. U.S. Pat. No. 5,357,374 (hereinafter, called as “D3”) proposes a zoom optical system of a negative-positive-negative three-unit type, wherein all the lens elements of the optical system are made of a plastic material for realizing a further compact and inexpensive optical system. A further recent technology achieved a zoom optical system of a negative-positive-positive three-unit type, which is conceived to be most advantageous for miniaturizing the optical system. An example of the advanced technology is recited in Japanese Unexamined Patent Publication No. 2003-177314 (hereinafter, called as “D4”), which is directed to a compact zoom optical system having a less number of lens elements ranging from two to four. Further, U.S. Pat. No. 6,735,020 (hereinafter, called as “D5”) proposes a zoom optical system, wherein a half or more than half of the total number of lens elements are plastic lens elements in an attempt to improve productivity of the optical system.
An example of an optical system other than the aforementioned optical system to be incorporated in an imaging lens unit is recited in U.S. Pat. No. 5,132,838 (hereinafter called as “D6”). D6 is directed to a compact zoom viewfinder optical system having objective lens groups of a negative-positive-positive three-group type. The objective optical system adopts typical movements of lens groups in a zoom optical system of a negative-positive-positive three-unit type, wherein the first lens group makes a u-turn, the second lens group is linearly moved spaced apart from the first lens group by a certain distance, and the third lens group is fixed (unmoved) during zooming from the wide-angle end to the telephoto end.
It is difficult to perform various aberration corrections in the two-unit type zoom optical system as proposed in D1 or D2, and it is difficult or impossible to apply the two-unit type zoom optical system to a recently developed image sensor with a large number of pixels. Furthermore, in D1, although all the lens elements are made of a plastic material, the entire length of the optical system is long because of the two-unit arrangement, and the moving distances of the first lens group and the second lens group in zooming are also large, which makes it difficult to miniaturize the entire construction of the imaging lens unit including the optical system. Since the optical system proposed in D3 has an arrangement that each of the lens groups is composed of a single lens element, aberration correction within each of the lens groups is insufficient, and the aberration of the optical system as a whole is large. Particularly, since the second lens group adapted for zooming has a strong optical power, constituting the second lens group of a single lens element is not desirable in correcting axial chromatic aberration. In addition to the demerit, the F-number is as large as about 7 to 10, which means less light is admitted. In D4, the first lens group and the third lens group each is composed of a single lens element, which results in insufficient aberration correction. In D5, the number of lens elements used in the optical system is as large as 7 or 8, in addition to the use of the glass lens elements. Thus, the entire length of the optical system is long, and there is room for improvement in terms of the size and the cost.
The objective optical system proposed in D6, which is an optical system other than the optical system to be incorporated in the imaging lens unit, has a feature that all the lens elements constituting the second lens group have a positive optical power, and accordingly, chromatic aberration correction is insufficient. Furthermore, since the entire length of the objective optical system is long, and distortion aberration correction is insufficient, it is difficult to incorporate the objective optical system in the imaging lens unit in the aspect of miniaturization and optical performance.
As mentioned above, the conventional zoom optical systems have suffered from the drawbacks that designing the optical system in an attempt to be compatible with an image sensor having a large number of pixels may unduly increase the number of lens elements and make the entire length of the optical system long. It is advantageous to use a glass lens element, particularly, an aspherical glass lens element in order to produce a compact and high resolution optical system. However, use of the aspherical glass lens element may increase the weight of the optical system and lead to production cost rise. Furthermore, high precision in processing is required in fabricating the aspherical glass lens element, which is not feasible in mass production. Additionally, the following matter should be considered. Generally, the moving distance of a lens group in a zoom optical system is large, as compared with an auto-focusing system having a movable lens section as in the case of the zoom optical system. If the weight of the optical system to be driven by a lens driver is increased by the weight corresponding to the glass lens element(s), the size of the lens driver is increased in order to securely move the lens groups. It is desirable to use as many plastic lens elements as possible, considering the cost and the weight of the optical system. However, use of the plastic lens element may encounter many disadvantages such as small optical power, difficulty in miniaturizing the optical system, difficulty in chromatic aberration correction, and a large back focus variation with environmental temperature change. Furthermore, error sensitivity of the lens group adapted for zooming may be unduly increased, as the optical system is miniaturized. This is true in both of the optical systems with the glass lens element and with the plastic lens element. Accordingly, adjustment regarding the lens groups is required. Thus, the conventional art failed to achieve a compact and inexpensive zoom optical system which is compatible with an image sensor with a large number of pixels, and is capable of being built in a compact mobile phone or a PDA.